CLI feature gaps: env unset, app update, and independent disk partition sizes

[erfanium]

1. h8 set env can't unset/remove environment variables

h8 set env only merges -- there is no way to remove an env var once set. The only workaround is a full PUT via the API with the subset of envs you want to keep.

Proposal: Add a --delete or --remove flag:

h8 set env myapp --delete GITEA_WORK_DIR          # remove single var
h8 set env myapp KEY1=val1 KEY2=val2 --clear       # replace all envs

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2. No h8 app update command for arbitrary config changes

There is no generic way to update app config from the CLI. The only partial-update commands are set image, set env, set replicas, and set disk. Changing the command, args, probes, ingress annotations, etc. requires the web console or raw API calls.

Proposal: Add a generic update command:

h8 app update myapp --command "sleep 3600"          # override entrypoint
h8 app update myapp --probe /healthz                 # set readiness probe
h8 app update myapp --json '{"svc":{...}}'           # raw partial update

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3. h8 set disk can't set independent partition sizes

Disk partitions share a single size_in_Gi -- you can't say "config gets 1GiB, data gets 5GiB". The two partitions split the total with no way to enforce boundaries.

[erfanium]

For h8 app update: during the Grist migration I needed to change command, update 7 env vars, and toggle is_enabled — all in sequence. A generic h8 app update that does GET→merge→PUT would handle all of these atomically without the field-wiping danger described in #8 and #9.

Proposed syntax:

h8 app update grist --command "sleep 3600"
h8 app update grist --no-enable-ssl
h8 app update grist --add-env KEY=val --remove-env OLD_KEY

Could be implemented as a thin wrapper that always GETs the full app config, applies requested changes to the in-memory object, then PUTs. This eliminates the entire class of "forgot to include field X and it got wiped" bugs.